This invention relates generally to impedance matching networks for matching a source impedance with a load impedance.
A common goal in connecting a source of electrical power to an electrical load is to maximize the power transfer from the source to the load. This goal is met when the output impedance of the source, or generator, is equal to the complex conjugate of the input impedance of the load.
By way of brief background, in alternating current (ac) circuits, impedance has a resistive component, referred to as the real component, and an inductive or capacitive component, referred to as the imaginary component. In conventional complex number notation, an impedance Z is given by Z=R+jX, where R is the real component, X is the imaginary component, and j is an operator equal to the square root of -1. Impedances are said to be complex conjugates when their resistive components are equal and their imaginary components are equal in magnitude but opposite in sign. If a generator impedance is Z.sub.G =R.sub.G +jX.sub.G, then maximum power will be transferred to a load when the load impedance is Z.sub.l =R.sub.g -jX.sub.G. Another way of thinking of complex conjugates is in terms of vector quantities. A simple resistive impedance may be thought of as a vector with a phase angle of zero. A complex impedance has a magnitude and a phase angle. Impedances that are complex conjugates of each other have equal magnitudes, but phase angles of equal magnitude and opposite sign.
In many circuit applications, the source or generator impedance does not match the load impedance, and an impedance matching network may be connected between the source and the load. Basically, the function of the impedance matching network is to present to the generator an impedance equal to the complex conjugate of the generator impedance, and to present to the load an impedance equal to the complex conjugate of the load impedance. The matching network contains a number of interconnected inductors and capacitors, some of which are adjustable in value to achieve the desired result.
U.S. Pat. Nos. 2,611,030, 4,375,051 and 4,621,242 disclose impedance matching circuits which use variable capacitance elements or a multi-tap transformer to adjust the impedance of matching circuits. The variable elements are adjusted by motors or solenoids. A disadvantage of these designs is that the adjustable elements are not solid state, and therefore will have reliability problems and a relatively slow response time.
U.S. Pat. Nos. 2,884,632 and 4,951,009 disclose impedance matching circuits in which the variable impedance element is an inductor comprising a primary winding on a toroidal core of magnetic material. The impedance of this inductor is adjusted by a low frequency current in a secondary winding on the toroid; this low frequency current generates a magnetic field which partially saturates the magnetic material, thereby altering the inductance seen at the primary winding. While this design allows solid state manufacture, it has the disadvantage that transformer coupling between the primary and secondary windings reflects parasitic impedances from the secondary winding into the primary winding, thereby altering the impedance of the primary winding away from the desired impedance and generating undesirable high-frequency resonances.
It will be appreciated from the foregoing that there is still a need for improvement in the field of dynamically adjustable impedance matching networks. The need is particularly acute in the field of plasma processing, as used in the fabrication of semiconductor circuitry. When the electrical load is a plasma, the load impedance is dynamic and nonlinear, and changes as more power is coupled to it, and as other variables, such as gas pressure and composition, are changed. Therefore, although the load impedance may be measured or estimated, for purposes of adjusting a matching network to optimize power transfer, the load impedance will change whenever the network values are adjusted. Accordingly, a dynamically adjustable network is essential for efficiently coupling power to a plasma. The present invention provides an effective alternative circuit design to those described in the forgoing U.S. Patents, which avoids the difficulties with the prior designs discussed above.